Terminal and tap connections for resistance element



A. J. WRIGHT 3,405,382

Oct. 8,1968

TERMINAL AND TAP CONNECTIONS FOR RESISTANCE ELEMENT 2 Sheets-Sheet 1Filed Sept. 13, 1965 FIG. 1

FIG. 3

FIG. 2

INVENTOR. ALAN J. WRIGHT add/W ATTORNEY FIG. 8

Oct. 8, 1968 A. J. WRIGHT 3,405,382

TERMINAL AND TAP CONNECTIONS FOR RESISTANCE ELEMENT Filed Sept. 13, 19652 Sheets-Sheet 2 4a 811 f f 2a 8a 3 2 5 v V y T7- 1 A FIG. 5 4:1

FlG.4

FIG. 6

INVENTOR.

ALAN J. WRIGHT ATTOR NEY United States Patent "ice 3,405,382 TERMINALAND TAP CGNNECTIONS FGR RESISTANCE ELEMENT Alan J. Wright, Orange,Califl, assignor to Beckman Instruments, Inc., a corporation ofCalifornia Filed Sept. 13, 1965, Scr. No. 486,607 2 Claims. (Cl.338-323) ABSTRACT OF THE DISCLOSURE A terminal lead wire connection fora resistance element in the form of a deposited layer of resistancematerial on a non-conductive base member. The base includes a terminallea'd passage having at least two sections formed at an angle withrespect to each other and adapted to snugly receive an end portion of aterminal lead wire over which is deposited a conductive material forcon-ducting a current between the resistance layer and the terminal leadwire.

The present invention relates to electrical resistance elementsemploying deposited layer resistance materials and is more particularlydirected to lead wire connections to tap and end terminations of suchresistance elements.

The invention is primarily directed to resistance elemerits and devicesusing a resistance material which is applied as a layer or track on anon-conductive support or base. One such layer of resistance material,suitable for such devices, is formed of a mixture of glass and metalparticles and commonly called cermet resistance material. Another suchresistance material is the well-known conductive plastic material, whichis formed of a mixture of a powdered plastic and conductive metalparticles of carbon. The resistance element may also be formed of a thinfilm of deposited metal.

Cermet type resistance elements are formed from composite mixtures offinely divided glass particles and finely divided metal particles whichare applied to a non-conductive base member and 'fired to a temperaturebelow the melting point of the metal but suflicient to melt the glassparticles and to fuse the mixture into a conglomerate mass. In thesecermet mixtures, it has been found desirable to use noble metals whichtend to resist oxidation at the temperatures required to melt the glassand to fuse the mixture.

The fused cermet mixture produces a smooth continuous track or filmhaving a surface that is resistant to high humidity and fungus. The hardsmooth surface and its durability makes this resistance elementespecially applicable for use in variable resistance devices, such aspotentiometers or rheostats which utilize a movable contact that musttraverse over the surface of the resistance layer. A number of typicalresistance films and methods of making the same are described in US.Patents No. 2,950,995; 2,950,996 and 3,149,002 issued in the name ofThomas M. Place, Sr. et al. and assigned to the same assignee as thepresent invention.

In order to conduct an electrical current to and from the resistancetrack, end and tap connections or terminations are normally attached tothe resistance track. These are usually formed by means of a conductivematerial bonded to the substrate or base member and also bonded orintimately in contact with portions of the resistance track. The tap andend terminations may be formed of commercially available fired-on silverfilms which are applied as a paste to suitable portions of the basemember. After application, the film is fired to drive off the organiccarrier materials and to solidly fuse the material to form a conductivestrip.

3,405,382 Patented Oct. 8, 1968 The base member, with the resistancetrack and its associated end and tap terminations, may be mounted withina housing which is, in turn, supplied with suitable terminal posts orstuds for connecting the device into an electrical circiut. It isnecessary, therefore, to connect the terminal posts or studs of thehousing to the conductive strips, or end and tap terminations, depositedon the base member. This is sometimes accomplished by use of small leadwires which are connected at one end to the end termination on the basemember and at the other end to the terminal post or stud of the housingmember. It is difiicult to achieve a physically strong bond between thetermination wire and the end termination strip connected to theresistance track. These wires have been spot welded to the terminationstrip, but such welding operation is extremely difiicult due to thedifferent types of material involved and because of the thinness of thetermination strip. In order to further support the connection it hasbeen necessary to apply an epoxy bond over the termination pad and itsweld to provide a measure of support. The selection of an appropriateepoxy becomes especially critical under certain environmental conditionsdue to the difference in expansion between the epoxy and the basemember.

Accordingly, it is an object of the present invention to provide animproved arrangement for attaching lead wires to the termination stripsof a deposited layer-type resistance device.

It is another object of the present invention to provide for a depositedlayer resistance element an improved lead wire connection to the endterminations and taps which connection exhibits relatively greatphysical strength and excellent electrical characteristics.

Further objects and advantages of the invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize the invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

Briefly, in accordance with a preferred form of the present invention,there is provided a non-conductive base member having disposed thereon adeposited layer of resistance material which is in turn electricallyconnected with end termination strips and tap connections formed of asuitable conductive material also deposited on the surface of the basemember. In order to connect the termination strips and taps to terminalposts in a housing member, there are provided a plurality ofelectrically conductive lead wires firmly attached to the base member bymeans of terminal lead passages formed in the base member, the passageshaving a cross-sectional dimension adapted to snugly receive an endportion of the terminal lead wire. The end portion of the terminal leadwire is bent so that, when a tension force is applied axially to saidterminal wire, the terminal lead passage exerts a frictional forceagainst the surface of the end portion of the terminal wire therebyrestraining movement of the terminal wire and retaining the end portionof said end terminal wire in electrical contact with the terminationpad.

For a better understanding of this and further aspects of the invention,reference may be had to the accompanying drawings in which:

FIGURE 1 is a perspective view of a resistance element embodying thestructure of the present invention;

FIGURE 2 is an enlarged partial top view of a section of the resistanceelement of FIGURE 1 illustrating in greater detail the terminal wireconnection to the end termination;

FIGURE 3 is a partial cross-sectional view taken along line 33 of FIGURE2 illustrating the terminal wire passage and connection;

FIGURE 4 is a partial top view similar to that of 3 FIGURE 2illustrating in detail another embodiment of the invention;

FIGURE is a partial cross-sectional view taken along line 5-5 of FIGURE4;

FIGURE 6 is a partial top view of still another embodimen-tof theinvention;

FIGURE 7 is a partial cross-sectional view taken essentially along line7--7 of FIGURE 6; and

FIGURE 8 is a cross-sectional view of still another embodiment of theinvention.

Referring now to FIGURE 1 of the drawing, there is shown a resistanceelement of the type adapted for use in a variable resistance device. Theresistance element comprises a substrate base member 2 of non-conductivematerial which may be in the form of a circular wafer or disc. The basemember 2 is usually formed of an unglazed homogeneous, non-porous,ceramic material, such as steatite or alumina or other non-conductivematerial well-known in the art. While the illustrated resistance deviceis in the form of a circular disc of the type used in rotary single turnpotentiometers or rotary variable resis tors, it will be understood thatthe present invention is not limited to resistance devices of thisparticular shape and that the invention is equally applicable toresistance devices of rectangular shape or any other suitableconfiguration.

In conventional practice, the substrate or base member 2 is molded,fired and then ground or lapped to provide a smooth relatively fiatsurface 2a for supporting a resistance film or layer 3 thereon. Theresistance layer 3 forms a conductive path when an electrical current isapplied to the resistance layer through end terminations such as members'4 and 6 and tap member 7. As hereinafter used in the specification andclaims, the term conductor terminals or lead wire connections are meantto include both end terminal connections or center tap connections asare commonly employed in the resistor or variable resistor field.

The resistance element or layer 3 may be formed of a suitable resistancematerial, such as conductive plastic material having a carbonaceousmetal dispersed throughout the fused plastic, or of a cermet resistancematerial, comprising a non-conductive glass binder material havingminute particles of noble metal or metal alloy dispersed throughout theelement. Or the layer 3 may comprise a thin metal film conductor of thetype well-known in the art. Particular examples of cermet materials andtheir methods of manufacture are fully explained in the aforementionedPlace et al. Patents Nos. 3,149,002; 2,950,996; and 2,950,995 which areassigned to the assignee of the present invention.

When the resistance track is formed of cermet materials, a mixture ofglass and metal particles, forming the layer 3, are deposited on thesubstrate or base member 2 by any suitable operation well-known in theart, such as brushing, spraying, stenciling or silk screening. After thecermet film or layer has been deposited upon the surface 2a of the basemember 2, the base and layer are preferably permitted to dry incirculating warm air for a short period and then fired in a furnace orconventional ceramic kiln. The purpose of the firing operation is tofuse the glass particles of the material into a continuous glassy phasewith the metal particles being uniformly dispersed throughout thematerial, and without melting the metal particles or producing bubblesor blisters on the surface of the layer 3. A mixture of glass and metalparticles, after fusion at a temperature below that of the metalconstituents, forms a continuous element having a hard smooth glassysurface.

End terminations 4 and 6 or tap member 7 may be applied to the basemember 2 either before or after the application of the resistance layer3. The end terminations are preferably formed of a material such asfired-on silver paste and deposited in the requisite configuration onthe surface of the base member and fired thereon at a sufficiently hightemperature to convert the metallic paste into conductive strips ofmetal thermally attached to the base. These strips are formed inposition to be in electrical contact with the resistance track (if theresistance track is applied after the strips). One metallic paste usedto form these strip portions is formed of a combination of metal powderscomprising about 50% by weight, gold (Au) and about 40% by weight,platinum (Pt), together with about 10% by weight of a glass binder.

Preferably the conductive end terminations extend at least the entirewidth of the resistance track 3 and provide outwardly extending pads 4a,6a and 7a arranged proximate the resistance track but out of the path ofany movable electrical contact or wiper to be positioned thereover. Thepads 4a, 6a, and 7a provide a space for the connection of terminal leadwires through which current is applied to the resistance element fromthe terminals of the housing (not shown).

As will be hereinafter described, the present invention is directed toan improved design construction for attaching the terminal lead wires tothe end terminations. As will be seen in FIGURE 1, lead wires 8 and 9are attached to the termination pads 4a and 6a and a lead wire 11 isattached to the tap termination pad 7a. In order to produce a relativestrong attachment, the substnate is first prepared by forming an endtermination passage therein which is adapted to receive one end portionof one of the respective lead wires. In the embodiment illustrated inFIGURE 3, the passage comprises a hole 12 drilled or otherwise formed inthe substrate 2 which receives the end 8a of the lead wire 8 therein.The end portion or tip 8a is then placed through the unfired terminalpad 4a and the lead wire bent so that it extends outward generally inthe same plane as the surface 2a of the base member 2. Any tension forceon the lead wire causes the sides of the lead wire to engage the innersurface of the terminal lead passage 12 and produces a frictional forcerestraining movement of the end portion 8a of the lead wire.

In order to produce an extremely good electrically conductive bondbetween the lead wire 8 and the terminal pad 4a, it is preferable tocoat the end portion 8a of the lead wire with the same metal-glassmixture or silver paste as that used for the conductive terminal strip4. Wh n the unit is then fired to a sufficiently high temperature toconvert the metal-glass mixture or silver paste into a layer of metal,this coating forms a bond retaining the lead wire firmly in the passage12 and in good electrical contact with the end pad 4a of the terminalstrip or end termination 4. The lead wires 8, 9 and 11 are not onlybonded directly to the end pad portion of the terminal strips but alsoare physically restrained by the end termination passage and bondedtherein by the coating material. As may be seen in FIGURES 2 and 3, inthis embodiment of the invention the terminal lead passage 12 isarranged at sub stantially a right angle to the surface 2a of the basememher 2 and the lead wire 8 is bent at substantially a right angle asit exits from the lead Wire passage. After the unit is fired, the leadwire 8, termination pad 4a, and covering material 13 are integrallybonded to the base 2 and the strip 4 is arranged in contact with theresistance element 3 which may have been fired previously or may befired thereafter.

Referring now to FIGURES 4 and 5, there is provided another embodimentof the invention in which the termination lead passage is in the form ofa semicircular groove 13 formed in the surface of the substratebasemember 2. The groove is substantially the same diameter as the lead Wire8 and provides a curved seat for the end portion 8a of the lead wire.This lead wire connection is constructed, as in the previously describedembodiment,

by first coating the end portion 8a of the lead wire 8'with a conductivematerial such as the aforementioned metal glass mixture, and bending thelead Wire 8a to-the con figuration of the semicircular lead Wire passage13. The end 8a of the lead wire is then inserted into the passage andthe end termination pad 40 is then deposited thereover. The unit is thenfired to a temperature sufficient to form a continuous metal conductor.As in the previously described embodiment, the resistance element 3 maybe deposited on the substrate base prior to or after the connection ofthe lead wire to the end termination.

In this embodiment of the invention, it is unnecessary to bend the leadwire 8 at right angles to make it emerge along a line substantiallyparallel to the surface 2a of the base member. Because of the curvatureof the passage 13, the lead wire engages the sides of the passage andthe coated portions of the lead wire are firmly attached or bondedthereto as well as being in good electrical contact with the end pad 4a.

Referring now to FIGURES 6 and 7, there is illustrated a combination ofthe lead wire connection concepts of FIGURES 2, 3, 4 and 5. In thisembodiment of the invention, the lead wire passage is in the form of asemicircular groove 14 which terminates in a hole 16 formed at an angleto the surface of the base member 2. While, the hole 16, in theillustrated embodiment, is at right angles with respect to the remainingportions of the passage 14, it could be disposed at any desired angle inorder to facilitate the assembly of the lead Wire.

In this embodiment the tip of the end portion 8a of the lead wire isformed over to fit into the hole 16, or angularly disposed portion ofthe passage, and the remaining portion of the end portion 8a is bent toconform to the semicircular groove 14. As in the previous embodiments ofthe invention, it is preferable to coat the end portion 8a of the leadwire with a conductive bonding material. Prior to firing, the pad 4a isdeposited over the surface 2a of the base member and over the passagecontaining the end portion 8a of the lead wire. The unit is subsequentlyfired to a temperature sufficient to drive off the organics in theconductive material and to fuse the conductive material and terminalwire into an electrically conductive connection. The conductiveresistance track 3 may be deposited on the surface of the base member 2either prior to or after the formation of the end termination pad andthe attachment of the lead Wire.

FIGURE 8 illustrates another embodiment of the invention in which thelead wire passage is formed by two holes 17 and 18 drilled or formedthrough the base member 2 and connected by a groove or passage 19 formedin the surface of the base member opposite from that upon which theresistance material 3 is deposited or is to be deposited. In thisembodiment of the invention, the paste material for the terminal pad 4ais preferably deposited on the surface 2a of the base member 2 prior tothe insertion of the end portion 8a of the lead wire 8. The end 8a ofthe lead Wire is coated with a suitable conductive material and insertedthrough the hole 17 of the passage. The end portion of the lead wire isthen reversely bent with the tip portion 8b of the lead wire extendingupwardly through the hole 18 of the passage. As shown in the drawing, itis preferable to have the end or tip 8b of the lead wire in contact withthe end pad 4a, however, this is not absolutely essential inasmuch asthe lead wire does contact the end pad 4a at its point of emergence 8c.The unit is then fired to a temperature sufficient to drive off theorganics in the conductive mat rial and to form a metallic bond betweenthe conductive material and the lead Wire 8. The coating on the endportion 8a of the lead wire forms a bond between the lead wire and thesides of the passage thereby firmly holding the lead wire in placeagainst any tension forces applied to the lead wire.

These fired-on lead wires have much more strength than a similar leadwhich is spot welded or soldered onto the terminal pad. The lead Wireconnection also provides a much better electrically conductive pathbecause of the greater length of engagement between the wire and thetermination pad. Because of the strength of the bond th connection doesnot have to be covered by an epoxy material in order to support the bondand retain the Wire in place.

Because the lead wire does not have to be spot heated at the point whereit fuses with the end termination pad, its size can be greatly increasedbecause the heat flow away from the bond point is no longer a criticalfactor. A larger diameter wire greatly facilitates the connection andincreases the strength of the unit. The larger diameter wire alsogreatly facilitates the connection of the lead wire at its opposite endsthereof to the electrical terminal studs (not shown) -of the housingmember in which the unit is mounted.

While in accordance with the patent statutes there has been providedwhat at present are considered to be the preferred embodiments of theinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made therein without departing from theinvention and it is, therefore, the aim of the appended claims to coverall such changes and modifications as fall within the true spirit andscope of the invention.

What is claimed is:

1. A lead wire connection for a deposited film type resistance elementor the like comprising:

a non-conductive base member having disposed thereon a resistanceelement formed of a deposited layer of resistance material;

a terminal lead passage formed in said base member proximate saidresistance element, said terminal lead passage including a first sectionin the form of a curved groove having one end thereof communicating withthe edge of said base member and a second section in the form of a holein said base member arranged at an angle with respect to said groove, anelectrically conductive terminal lead wire having an end portion thereofdisposed in said sections of said terminal lead passage and bent toconform to the configuration thereof so that when a tension force isapplied axially to said terminal wire said passage exerts a frictionalforce against the surface of said end portion of said terminal Wirerestraining movement of said terminal wire; and

. a layer of conductive material deposited on said base member over saidterminal lead passage and said terminal wire disposed therein, saidlayer of conductive material bonding said terminal lead wire into saidpassage and being in electrical contact with said resistance element forconducting current between said resistance element and said terminallead wire.

2. A lead wire connection for a deposited film type resistance elementor the like comprising:

a non-conductive base member having disposed thereon a resistanceelement formed of a deposited layer of resistance material;

a terminal lead passage formed in said base member proximate saidresistance element, said terminal lead passage comprising a groovehaving a shape substantially in the form of a semi-circle formed in saidsurface of said base member, one end of said groove communicating withthe edge of said base member;

an electrically conductive terminal lead wire disposed in said groove,said groove having a cross-sectional dimension adapted to snugly receivean end portion of said lead wire with the remainder of said lead wireprojecting from said groove along a line substantially parallel with thesurface of said base member, said terminal lead wire being bent toconform to the semi-circular shape of said gnoove so that when a tensionforce is applied axially to said terminal lead wire said passage exertsa frictional force against the surface of said end portion of saidterminal wire restraining movement of said terminal wire; and

a layer of conductive material deposited on said base member over saidterminal lead passage and said terminal wire disposed therein, saidlayer of conductive material bonding said terminal wire into saidpassage and forming an electrical contact with said resistance elementand said terminal lead wire for conducting current between saidresistance element and said terminal lead wire.

References Cited UNITED STATES PATENTS 3,117,298 1/1964 Grunwald 3383 173,202,95 1

OTHER REFERENCES Koelsch, Jr., A. 0., Methods of Connecting Wires toSubstrates, IBM Technical Disclosure Bulletin vol. 3 No. 2, July, 1960,p. 72.

RICHARD M. WOOD, Primary Examiner.

8/1965 Krinsky 338 -308 X 10 J. G. SMITH, Assistant Examiner.

